This invention relates to microwave circuit modules and, more particularly, to an antenna array that is relatively efficient, is relatively inexpensive to manufacture, may provide electronic beam steering, is relatively light in weight and may be more aesthetically pleasing than other kinds of antennas.
Phased array antennas are well known for transmitting and receiving microwave radiation for information communication. An early example of a phased antenna array is described in U.S. Pat. No. 2,622,198. An advantage to such phased antenna arrays is the relative ease in "steering" the beam transmitted or received by that array, thereby providing good directivity to improve the transmission/reception qualities of the antenna array. Such directionality of the microwave beam is controlled electronically and, thus, the antenna may be easily "steered" to a desired transmitting antenna. Such electronic beam steering of a phased antenna array is particularly useful in satellite communication.
Phased array antennas generally are relatively expensive; and the overall construction of such arrays has resulted in relatively heavy structures which are somewhat difficult to handle, assemble and position in desired locations at preferred sites. Consequently, it is desirable to form such antenna arrays of materials that would avoid these problems. Nevertheless, typical antenna arrays at the present time are still relatively expensive.
U.S. Pat. No. 4,038,742 describes a styrofoam slotted plane-array antenna. This antenna is formed of HD-300 styrofoam which has a loss tangent of approximately 0.0004 and a relative dielectric constant of 1.07. This styrofoam is a closed cell material which is easily machined to a relatively smooth surface. The antenna is formed by depositing a layer of copper on the surface of the rectangular waveguide section of the styrofoam and then forming separate sections by plating through holes which form short circuits within the resultant waveguide. The waveguide then has a series of radiating slots milled in one metallized surface so as to function as a phased array antenna. Separate sections formed in the aforementioned manner are bonded together to form a multi-section array. The manufacturing costs of combining the various sections of the waveguide and the amount of copper used make the cost of manufacturing the overall antenna array relatively expensive. Moreover, the antenna array is relatively heavy and bulky.
Reissue Pat. No. 29,911 describes a microstrip antenna in which an array of radiator elements, known as microstrip patches, are formed by etching away conductive material that coats one surface of a dielectric substrate. The opposite surface of this substrate has a conducting ground plane deposited thereon. RF feedlines interconnect the microstrip patches; and phase shifting arrangements are used such that the radiator elements emit circularly polarized radiation. Depending upon the particular shape of the patch and the manner in which the feedline is connected to that patch, different resonant frequencies and different polarizations can be attained. The resultant antenna can be used in the X-band; and it is believed that the teachings of this patent also are applicable to other conventional frequency bands. There is no clear disclosure in this patent of the structure or materials which constitute the aforementioned dielectric substrate.
So-called suspended strip line (SSL) antennas have been proposed for compact phased array antennas, as described in the March 1984 publication Microwave Systems News. The SSL antenna uses an SSL feed network; and the antenna has an open waveguide transition. According to this publication, the SSL antenna is composed of two metal plates, each with openings corresponding to the antenna elements, and the feed network includes a central conductor supported by a thin dielectric sheet. The SSL antenna achieves circular polarization by using two superimposed networks rotated 90.degree. with respect to each other. The two orthogonal outputs are combined through a 3db hybrid coupler; and left-hand circular polarization as well as right-hand circular polarization can be achieved. Electronic beam steering is attained through an appropriate phase control in the feed network.
It also has been proposed in a report dated March 1980 by Ingmar Karlsson, of L. M. Ericsson of Sweden, to support an antenna by a foam whose thickness is about 0.15 meters and which is fed by a separate microstrip network. The antenna is formed of rectangular microstrip patches which are fed from behind and in parallel from the separate strip line network. According to this report, the microstrip feed network and the radiators can be etched on the same substrate. The report also describes an antenna in which the microstrip patches were etched on a thin GFRB substrate which was supported 14 mm above a ground plane by a low density Divinycell foam; and the microstrip feed network was etched on the other side of that foam. However, the requirement of etching the microstrip feed network and the microstrip patches adds significantly to the overall cost of this antenna.
It is believed that there has been a long felt need for an antenna, and more generally, for microwave components, which may be formed as lightweight structures and may be manufactured relatively inexpensively and quickly. The overall process of etching, as required in the aforementioned SSL antenna, the Karlsson report and Reissue Pat. No. 29,911 is expensive and time-consuming. It had been thought heretofore that the direct application of conductive patterns, forming radiator elements, feed networks, phase shifting circuits, and the like, on the surface of a low cost polyethylene foam, could not be achieved. Even if such conductive patterns could be deposited on such a foam, it had been thought that a satisfactorily operating antenna could not be formed thereby.